1. Field of the Invention
The present invention relates to an aberration detection device for an optical system used for an optical information recording and reproducing apparatus for recording information on an optical information recording medium (also referred to as “information carrier” in the following), such as an optical disk, and/or reproducing recorded information.
The present invention also relates to an optical information recording and reproducing apparatus for recording large amounts of information on an optical information recording medium (information carrier) with laser light, and for reproducing the recorded information. This aspect relates in particular to an optical information recording and reproducing apparatus for an information carrier having a plurality of information recording layers, such as an optical disk.
2. Description of the Prior Art
First Aspect
A conventional aberration correction system for optical disks is published in Publication of Unexamined Japanese Patent Application (Tokkai) No. Hei 8-212611.
FIG. 20 is a diagram of such a conventional wavefront aberration correction method. In FIG. 20, numeral 801 denotes an optomagnetic disk, numeral 811 denotes a semiconductor laser, numeral 812 denotes a collimator lens for collimating the divergent light bundle emitted by the semiconductor laser 811 into a parallel light bundle, numeral 813 denotes an anamorphic prism for correcting the light bundle into a light bundle with circular cross section, numerals 814 and 816 denote reflecting mirrors, numeral 817 denotes an object lens, and numeral 818 denotes a liquid crystal element. Moreover, numeral 820 denotes a complex prism, numeral 822 denotes an APC sensor for detecting and controlling the power of the laser light, numeral 825 denotes a λ/2 plate, numeral 826 denotes a polarization beam splitter, numerals 829, 830, and 833 denote light receiving elements, numeral 850 denotes a liquid crystal control circuit, and numeral 854 denotes a microcomputer.
In the device in FIG. 20, the liquid crystal control circuit 850 is driven based on data from a memory to control the liquid crystal element 818 so as to perform aberration correction. In particular, when aberrations occur, the phase of the liquid crystal aberration correction element 818 is controlled by an open loop, so that the wavefront aberration becomes minimal. Also, to correct wavefront aberration changes due to temperature influences, the temperature is detected, and the wavefront aberration is corrected on the basis of the detected temperature and previously stored control data relating to the temperature.
In the example in FIG. 20, the signals from the light receiving elements 829 and 830 for signal detection and the light receiving element 833 for error signal detection are entered into the microcomputer 854, which determines the voltages that the liquid crystal control circuit 850 applies to the elements of the liquid crystal element 818, so that the detection signal of the light receiving elements is improved.
A method for detecting aberration disclosed in the same publication measures the wavefront aberration with an interference system. Moreover, after determining the disk type and the necessary data for controlling the liquid crystal so as to correct the wavefront aberration occurring when that disk type is used, the correction of the wavefront aberration is performed based on a pre-arranged table. To do so, a measurement device comprising an interference system is arranged on the outside to measure the wavefront aberration, but the publication does not disclose a specific configuration of the interference system.
To optimize the S/N ratio with these conventional aberration correction methods, the wavefront aberration is changed by trial and error, and a closed loop is formed that minimizes the wavefront aberration as a result.
However, judging with these methods whether the signal improves or deteriorates, the determination of the optimal point becomes tedious (i.e. trial and error), so that the detection takes time and it is not possible to perform control with a closed loop with fast response.
Second Aspect
Types of so-called read-only optical information recording media that reproduce signals using laser light include compact disks (CDs), laser disks (LDs), and digital video disks (DVDs).
Presently, the read-only optical information recording medium with the highest signal recording density on the market is the DVD-ROM with 4.7GB.
Standardized formats for read-only DVDs with a diameter of 120 mm include the single-side single-layer type with 4.7GB maximum user capacity, the double-side single-layer type with 9.4GB maximum user capacity, and the single-side double-layer type with 8.5GB maximum user capacity.
FIG. 21 shows an example of the structure of a single-side double layer optical disk. In this optical disk, by irradiating a laser beam from the side of a substrate 918, signals recorded in either a first information recording layer 919 or a second information recording layer 921 can be reproduced through the substrate 918. Between the first information recording layer 919 and the second information recording layer 921, an optical separation layer 920 is provided, which optically separates the laser light entering through the substrate 918 to the first information recording layer 919 and the second information recording layer 921. Below the second information recording layer 921, a protective substrate 922 for protecting the second information recording layer 921 is provided. A method for manufacturing such a multi-layered read-only optical disk is disclosed, for example, in U.S. Pat. No. 5,126,996.
Moreover, types of optical information recording media on which a signal can be recorded and reproduced using laser light include phase-changing optical disks, optomagnetic disks, and dye disks.
In recordable phase-changing optical disks, a chalcogen compound is normally used as a material for the recording thin film. Usually, the crystalline state of this recording thin film material is regarded as the unrecorded state, and signals are recorded by irradiating laser light and changing the recording thin film material into the amorphous state by melting and cooling it quickly. Conversely, to erase signals, laser light is irradiated at lower power than for the recording, and the recording thing film is crystallized.
As an attempt to increase the recording density of recordable or recordable/erasable optical disks, the so-called “land & groove recording” has been proposed (see for example Tokkai Hei 5-282705), wherein signals are recorded in both the guide grooves and the guide lands provided in a substrate surface.
Moreover, as an attempt to increase the recording capacity of recordable or recordable/erasable phase-changing optical disks, double-layer disks have been suggested (see for example Tokkai No. Hei 9-212917).
To raise the recording/reproducing density of these disks, it is desirable to perform recording and reproducing with an object lens that has a high numerical aperture (NA). Among conventional optical disk devices, there is no example of a device using an object lens with a NA that is high enough so that errors in the thickness of the substrate may have become a problem, and irregularities in the substrate thickness have not been a particular problem.
An idea of how to correct spherical aberrations of a double-layer disk with the reproducing apparatus is mentioned in Tokkai Hei 7-77031. In this publication, a predicted aberration amount of spherical aberration that occurs when using an object lens and a double-layer disk is corrected. As an element for generating an optical phase difference to correct the aberration, a liquid crystal layer is mentioned in an example embodiment. For low NAs, this method provides sufficient correction.
This means, even when the disk substrate is produced with high precision, there are still thickness irregularities of normally 30 to 60 μm, and the thickness irregularities for CDs are about 100 μm. To reproduce a CD, a lens with an NA of 0.4 to 0.45 is used. In the case of a device for recordable CD-Rs, a lens with an NA of about 0.5 is used. In the case of DVDs, a lens with a NA of 0.6 is used, because of the high density of the DVD. For disks with thickness irregularities in the range of about 30-100 μm, acceptable recording and reproduction can be performed with recording/reproduction system having a NA of not more than 0.6. However, when the NA is more than 0.6, the thickness irregularities of the substrate and the aberrations intrinsic to the lens itself become a problem.
With the method disclosed in Tokkai 7-77031, it is not possible to correct the spherical aberrations that occur when the thickness of the substrate changes. Moreover, because the correction element is arranged within the optical system, the spherical aberration correction element has an optical axis that is different from the optical axis of the object lens, so that the spherical aberration, which varies in proportion to the fourth power of the NA, becomes large, and this method becomes unsuitable for optical systems with a high NA.
The idea of doubling the recording capacity of recordable/erasable optical disks with a double-layer structure already has been proposed (see, for example, Tokkai Hei 9-212917), but since a method solving the following problems has not yet been found, it has not been put into practice. In the present invention, “first information recording layer” means a first recordable layer, seen from the side where the laser light for recording and reproduction enters the recording medium, and “second information recording layer” means a recordable layer behind the first information recording layer, seen from the side where the laser light for recording and reproduction enters the recording medium. In particular, those problems are:
1. No means has been found for performing recording and reproduction with the same suitable level for both the first and the second information recording layer, using an object lens with high NA in the optical system for recording, erasing and reproducing signals.
2. No means has been found for reducing spherical aberration for both the first and the second information recording layer, using an object lens with high NA in the optical system for recording, erasing and reproducing signals.
3. No configuration for an optical system that can overwrite the first and the second information recording layers at high speeds has been found.
An optical information recording medium in accordance with the present invention comprises a first information recording layer, an optical separation layer, a second information recording layer, and possibly more information recording layers, each two neighboring information recording layers being separated by an optical separation layer, formed in this order on a substrate. The information recording layers comprise a material with which information can be recorded and reproduced. Typical materials for the information recording layers are recording materials, in which a reversible phase-change between an amorphous state and a crystalline state can be caused by irradiation with laser light, so that signals can be recorded, erased or reproduced by irradiation with laser light through the substrate.
If recording and reproducing is performed with an optical disk having such a substrate, aberration occurs depending on how much the actual thickness deviates from the design thickness of the substrate used for designing the lens (in the following also referred to as “substrate design thickness”).
When the deviation of the substrate thickness from the substrate design thickness is t, the refractive index of the substrate is n, and the numerical aperture of the object lens is NA, then the spherical aberration W40 generated at this NA can be expressed byW40=(1/8)(1/n−1/n3)t(NA)4
When this aberration exceeds 35 mλ (millilambda), wherein λ is the operation wavelength, it adversely affects the recording and reproduction characteristics considerably.
For example, if NA=0.60, n=1.5, and W40=35 mλ, then t=14.5 μm.
Considering, for simplicity, a double-layer disk having two information recording layers, if the substrate design thickness is just about half the width of the double-layer disk, then the maximum change in the thickness is ±14.5 μm, so that the thickness between the two layers has to be less than 29 μm. If, however, the thickness between the two layers is small, then the interferences between the layers become large, which adversely affects the recording and reproduction properties. For example, assuming that the distance between the layers is about 10 μm, stray light from one layer influences the focus servo for recording/reproducing the other layer, so that it is not possible to perform adequate recording and reproduction.
Consequently, a thickness between the layers that is tolerable in practice is 15 μm to 29 μm, but to actually manufacture such a disk leads to considerable difficulties.